Configurable 2D nano-flows in mesoporous films using paper patches

Designing and controlling spontaneous imbibition is becoming a key requirement for advanced devices, presenting a substantial scientific and engineering challenge. Here we describe an approach that allows directional imbibition into designed geometries. A set of custom domains based on paper microfluidics mold nano-imbibition in user-defined shapes such as curvatures, corners, and vertices into mesoporous thin films; enabling localized chemical reactions with programmable designs. The method also achieves nano-size filtration, allows the generation and delivery of reagent gradients in a nanofluidic fashion, and it can be used as a reactor for the synthesis of patterned metallic nanoparticle arrays. By using this easy-to-build hybrid platform, users can create functional nanofluidic domains in custom geometries and perform spatially shaped chemistry. The ability to integrate mesoporous nanofluidic generation and paper-based microfluidics has made the hybrid system an exciting candidate for versatile nanoflow applications.


Introduction
Spontaneous imbibition enables the propelling of ows because of the preponderance of capillarity at small length scales. 1 One challenge in ow dominance is the design of imbibition to attain total control of the arrangement of uids. Such uidic transport attracts interest from the elds of fundamental research, medicine, and biotechnology. Currently there is increasing activity in developing microuidic paperbased devices, 2,3 where passive pumping represents a practical advantage added to its extraordinarily low cost and compatibility with most (bio)chemical reactions. Paper-based micro-uidics provides access to functional modules with overall or repeat unit dimensions ranging from tens of microns to centimeters. 4 On the other hand, mesoporous lms exhibit arrested uid imbibition due to the balance between capillary lling and surface evaporation to the environment. 5 This effect allows one the implementation of mesoporous lm-based nanouidics, which offers the possibility to manipulate ultrasmall amounts of liquid, 6 and also serves as hubs for forming high-localized reactions. 7 An additional advantage of this strategy is that the usually costly steps of sealing and tubing can be obviated in the fabrication of nanodevices. With the idea in mind to generate addressable nano-imbibition with increasing patterns of complexity and controlled reagents delivery, we have used mesoporous lm based functionalities into designed paperbased scaffolds. In this study, we have achieved reliable design of the two dimensional imbibition of mesoporous lms with programmed control over their geometry. The capabilities of this method are demonstrated by implementing chemical reactions in shaped uidic domains, including nano-optical structure synthesis; attaining hierarchical selectivity in micro/ nano ltration; and generating chemical gradients and subsequently unloading reagents to achieve preprogrammed reactions. Therefore, these on-demand imbibitions behave as smart nano-ows which provide a well-founded starting point for the exploration of more sophisticated chemical and biomedical applications.

Shaped nanouidic domains
Controlling the spatial arrangement of spontaneous inltration in nanoporous systems is a central goal for versatile devices. We then reimagined the open pit design principle 7 from paperbased microuidics. Instead of using sessile drops as reservoirs, now our approach uses imbibed papers. In this method, a paper with arbitrary shape assumes the role of a reservoir that is cut into a user-dened arrangement (sketched in Fig. 1). We explored the possibility of creating custom curvatures, corners, and vertices, in order to copy these forms into the uid front of the liquid that inltrates the underlying material. Our devices are based on a mesoporous oxide lm grown on silicon or glass substrates using F-127 as a surfactant template, by combining sol-gel chemistry and evaporation-induced self-assembly. 5,8 Fig. S1 in ESI † shows typical SEM and TEM images of the mesoporous silica and titania lms used in this work, where the nanopores can be clearly observed.
We then placed a series of paper shapes, a 180 straight front, an open concavity and a triangle on the surface of mesoporous lms. When a drop is deposited on the paper the liquid from the paper reservoir enters the porous substrate and builds a halo at its periphery. This halo nucleates and grows via capillarity at their free boundaries which inltrate into the nanoporous lm. Imaging on each preformed paper corroborated the successful imbibition with the designed shape (see Fig. 2). The wetted region can be clearly seen because it produces a refractive index contrast in relation to the dry zone. 5 The at paper limits were more reproduced than the corners, as expected because of the paper adhesion constraint in the vertices. Together, the different shapes in Fig. 2 constitute a set of design motifs that could be combined to create larger, more complex patterns. It is worth mentioning that the hybrid system is modular and scalable, and can therefore be extended from the micrometers to centimeter scale according to the surface/ area of the cut paper.

Size-exclusion capabilities
In order to take advantage of the size-exclusion properties of the nanopores of our hybrid uidic platform, we incubated a 5 mm in diameter paper-reservoir deposited on a silica mesoporous lm with a mixture comprising an AgNO 3 solution and iron oxide nanoparticles (nanocrystalline magnetite, Fe 3 O 4 with an average size of 10 nm; leading to a narrow nanocluster size distribution in aqueous solution of ca. 35 nm. See ESI † for more details). 9 Aer mesoporous inltration the paper was removed and Energy Dispersive X-ray Spectroscopy (EDS) was used to quantitatively measure the presence of Ag and Fe in the different zones. The EDS data obtained from the paper and mesoporous zones revealed the presence of both Fe and Ag along the paper region and only Ag ions in the inltrated mesoporous outside the paper zone (see Fig. 3). It is evident that the paper released all two species while the mesoporous lm avoids the transport of the magnetite nanoparticles and enables the diffusion of Ag ions into the lm, indicating that the system can lter as designed. The relatively large Fe 3 O 4 nanoclusters are excluded from the small pore silica which however can act as a host layer for smaller entities such as Ag ions. This example illustrates that the synergic properties of paper-based micro-uidics integrated with a mesoporous lm solve the challenge of simultaneously achieving size selectivity and tuned nanoow. With the wealth of mesoporous structures developed is possible to design the selectivity of these hybrid systems by varying the pore dimensions in the mesoporous lm.

Molded chemistry
We further capitalized on this new hybrid system by expanding their capabilities to form localized reactions with programmable geometries. We then tested the possibility to localize a chemical reaction with user-desired shapes starting from custom shaped cut papers with a separating gap between them. To this end, we used the well-known Ag + + Cl À / AgCl reaction by depositing microliter drops of 1 M sodium chloride (NaCl) and 0.1 M silver nitrate (AgNO 3 ) on both paper strips, respectively. 7 Fig. 4 shows the proper localization of silver chloride   (AgCl) precipitation through the nanocapillary communication of the different paper-reservoirs, and more importantly demonstrating the capability of the hybrid method for molding chemical reactions at the nanouidic connection. The large distances over which these shaped reactions extend demonstrate the overall robustness of this hybrid method. Although the method was designed to generate pointed structures formed by chemical reaction at the time of contact, we expected to (and nally did) observe an extended rounded precipitation shape because of the front roughening effect that takes place during spontaneous imbibition in porous media. 7 Taking advantage of the latent properties described above for "drawing" with metallic nanoparticles, we used the process for transferring geometrical contours on a cut paper into gold nanoparticle arrays. Optical images in Fig. 4b display the creation of nanostructured gold patterns with arbitrary shape via salt reduction by depositing microliter drops of 0.1 M tetrachloroauric (HAuCl 4 ) at pH ¼ 4, and 1 M sodium borohydride (NaBH 4 ) in each paper reservoir, respectively. The characteristic ultravioletvisible spectrum (surface plasmon resonance) evidences that the precipitate is an extended nanoassembly of Au nanoparticles (see inset in Fig. 4b). 10 Further, these reaction structures also form well-dened substructures that run parallel each other. This capability will allow for applications that require a precise spatial location of metallic nanospecies such as in advanced catalysis, nanoelectronic circuits, chemical and biological sensor devices, optical switches, etc. 11 Additional investigation of the substructuration mechanism will prove useful in the development of this method for patterning complex opto-electronic nanostructures.

Nanouidic gradient-actuated reactions
Finally, in order to test the utility of the hybrid method for controlling the concentration of solutes across the mesoporous membrane, we conducted a pH gradient experiment by introducing a paper-based gradient generator. The simple format of a uniform strip with Y-shaped inlet ports was employed. 12 Besides, here the system was laminated to avoid evaporation, leaving both ends (inlet ports and gradient exit) open to the atmosphere. Pure water and NaOH 1 M solution are added to each one of the inlet arms. The capillary-driven co-ow of solutions along the strip enables the dispersion of OH ions from the concentrated stream to the other, leading to a linear gradient that ranges from 7 to 14 at the strip end, perpendicular to the ow direction. This strip end was in contact with the mesoporous lm, as shown in Fig. 5 (le panel). The use of such basic solutions to precipitate oxides is a long-standing practice in the eld of material science. 13 It should be mentioned that oxide precipitation in metal salt solutions is produced in high pH conditions. Here, we adapted this method to test a localized gradient in pH into mesoporous lm. We then performed the well-known 2 AgNO 3 (aq) + 2 NaOH(aq) / Ag 2 O(s) + 2 NaNO 3 (aq) + H 2 O(l) reaction and analyzed the response of the  precipitation signal to formation of the pH gradient by the paper system. In fact, this precipitation was actuated by imposing a pH gradient along the nanouidic contact interface. As expected, this assay produced a precipitation reaction within the interface only in the high pH areas (see Fig. 5). The analysis of optical microscopy images makes evident that precipitation depends on the concentrations of OH À in the nanopores, which has been regulated by using the paper-based pH gradient. These observations conrmed the proper graduation of OH À through the nanopores, thus demonstrating the utility of the hybrid method for producing space-selective solute concentration into the inltrated uid. Paper-based generation of concentration gradients could be used to direct the movement of cells 14 on the mesoporous substrates 15 or used for drug delivery by nanodispensers for dose testing.

Conclusion
We successfully developed a versatile multiscale uidhandling strategy based on nanopores-paper-scissors. Together the results here presented demonstrate that a hybrid platform of paper/mesoporous lm is able to mold uid transport and chemical reaction shapes with synergistic features such as size selectivity and the capability of creating gradients, even if the procedure has not been yet fully optimized. Although several types of nano-hybrid paper devices have been described, the most signicant difference with our proposal is that they are mostly obtained by modication of paper substrate, which demonstrates the high simplicity of the strategy here presented. We created a diversity of uid-fronts in the spontaneous mesoporous lm imbibition by designing custom paper shapes only using a cutter, and by means of these hybrid structures, we designed nano-size ltration and user-dened shapes for chemical reactions. Our approach for creating paper/mesoporous lm hybrid devices affords attractive capabilities with respect to positioning target features in custom nanostructured frameworks, and offers the additional advantage that all structural components are inexpensive.
Apart from the possibility for scalable mass production and the straightforward of exploiting nanopore functionalization [16][17][18][19] and connement, 20,21 our method could therefore enable novel applications that so far have not been amenable to current methods in paper-based microuidic. This heterogeneously integrated approach provide indeed synergetic functionalities to systems that could be important in various elds of application, including not only those suggested by the systems reported here but also others such as devices with integrated electronics, 22 chemical and biological sensor systems that incorporate unusual nanomaterials with conventional paper based microuidics, and photonic and optouidic porous silicon structures. 23 Furthermore, the compatibility of this approach with nano-optical detection from synthesis of metallic nanoparticles in arbitrary patterns may create additional opportunities for devices that have unusual form factors as key features. 24

Mesoporous thin lms synthesis and characterization
Crack-free mesoporous titania and silica lms were synthetized using sol-gel technology and evaporation-induced selfassembly technique. For this purpose, Pluronic F127 was used as the polymeric template. Titania lms were dip coated at 3 mm s À1 on silicon and glass substrates at 20% relative humidity, while silica lms were deposited on silicon at 1.5 mm s À1 withdrawal rate. Precursor solution for titania lms was composed of TiCl 4 : EtOH : H 2 O : F127 with a molar ratio of 1 : 40 : 5 : 0.005 and the initial solution for silica lms was the mixture TEOS : EtOH : H 2 O : F127 with a molar ratio of 1 : 40 : 5 : 0.04. Aer deposition, samples were calcined in air at 450 C for 10 min by a fast-ring process in order to remove the template. Samples were characterized with a Zeiss Leo 982 Gemini Scanning Electron Microscope (SEM) coupled to an Xray microanalyzer and using a Phillips CM 200 Transmission Electron Microscope (TEM).
Hybrid paper-mesoporous lm system Hybrid system was built with mesoporous titania and/or silica lms and Whatman No. 1 lter paper used as received. Paper strips with different geometries were shaped using a drawing cutter and directly deposited on the surface of the mesoporous lm. Molded chemical reactions for AgCl precipitation were performed using paper reservoirs imbibed of 20 ml of silver nitrate (AgNO 3 ) 0.1 M and 20 ml of sodium chloride (NaCl) 1 M, spaced 5 mm apart. In case of Au nanoparticles precipitation, 10 ml of tetrachloroauric (HAuCl 4 ) 0.1 M at pH 4 and 20 ml of sodium borohydride (NaBH 4 ) 1 M were used. UV-visible spectrum was obtained using a UV-1800 Shimadzu Spectrophotometer. Precipitation zone in microscope areas and the studies of the imbibition fronts were followed by a Mitutoyo FS70 microscope. Images were recorded using a high-resolution digital camera. Nanouidic gradient-actuated reactions were carried on using a uniform strip with Y-shaped inlet ports. In this case, the paper was laminated to avoid evaporation, leaving both inlets and the gradient outlet open to atmosphere. To generate the pH gradient on the paper strip: 30 ml of NaOH 1 M and 30 ml of deionized water were added to each inlet arms. Then the system was brought in contact to the mesoporous lm and faced to a paper reservoir containing 10 ml of AgNO 3 0.1 M at pH 9. Gradient precipitation reaction was observed under the Mitutoyo FS70 microscope.

Conflicts of interest
There are no conicts to declare.